1. Field of the Invention
The present invention relates to receivers and the like having the function of detecting and eliminating an interfering wave adjacent to the desired reception wave.
2. Description of the Related Art
In conventional FM broadcasting receivers and the like, the function of detecting and eliminating an adjacent interfering wave near a desired broadcast wave is achieved by the configuration of a receiver as shown in FIG. 1.
In FIG. 1, a received electromagnetic wave from an antenna is subjected to processing such as tuning and high-frequency amplification in a high-frequency processing circuit 1, and then supplied to an intermediate-frequency processing circuit 2. The intermediate-frequency processing circuit 2 first converts the frequency of a high-frequency signal that has been selected through tuning in the high-frequency processing circuit 1 into a predetermined intermediate-frequency. Then, the intermediate-frequency processing circuit 2 passes the intermediate-frequency signal through a bandpass filter having a predetermined pass bandwidth, eliminates interfering waves adjacent to the desired reception wave, and outputs the signal to a detection circuit 3 in the following stage. The detection circuit 3 detects an output signal from the intermediate-frequency processing circuit 2 by using an FM detector such as a ratio detector, a quadrature detector, or a Foster-Seeley discriminator for example, and extracts an audio signal that is superimposed on the output signal.
On the other hand, a high-frequency noise detection circuit 6 passes the output signal from the detection circuit 3 through, for example, a high-pass filter having a predetermined cut-off frequency and detects high-frequency noise components contained in the audio signal of the detection output. The high-frequency noise components can be regarded as the result of signal components of interfering waves adjacent to the desired reception wave manifesting themselves in the detected output. Therefore, a control circuit 5 decides the presence of adjacent interfering waves from the detection result and controls the pass bandwidth of a bandpass filter (hereinafter, referred to as simply “IF-BPF”) built into the intermediate-frequency processing circuit 2.
That is to say, the wider the pass bandwidth of the IF-BPF is, the higher the sound quality of the audio signal that is superimposed on the desired reception wave through frequency modulation is, because sideband wave groups of the desired reception wave can be picked up broadly. But at the same time, there is the risk that many interfering waves adjacent to the desired reception wave are picked up. On the other hand, if the pass bandwidth of the IF-BPF is narrowed, adjacent interfering waves can be eliminated, however many of the sideband wave groups of the desired reception wave are eliminated at the same time.
The control circuit 5 therefore examines the size of the high-frequency noise components that have been detected by the high-frequency noise detection circuit 6, and when this noise level exceeds a predetermined value, it performs a control for narrowing the pass bandwidth of the IF-BPF from BW1 to BW2, for example, as shown in FIG. 2. In FIG. 2, the horizontal axis represents the frequency, and the vertical axis represents the signal level of the frequency spectrum.
In the conventional method, however, the frequency range in which there are adjacent interfering waves near the desired reception wave cannot be decided because the signal components of the adjacent interfering waves are deduced from the high-frequency noise contained in the output signal after detection. In addition, when multi-path noise occurs from a temporary change in the conditions for electromagnetic wave propagation, or when there is noise due to over-modulation distortion, then the signal components produced by that noise are also interpreted as adjacent interfering waves. Therefore, it has been difficult to effectively eliminate adjacent interfering waves by optimally controlling the pass bandwidth of the IF-BPF with conventional methods.
Therefore, the above problem is an example of the problems to be solved by the present invention. An object of the present invention is to provide a receiver with adjacent interfering wave eliminating function that allows to efficiently eliminate adjacent interfering waves without loss of sound quality of a receiving signal.